The present invention relates to a metal tube oxidation treatment apparatus, and relates particularly to a metal tube oxidation treatment apparatus for carrying out the inactivation treatment of the metal tube used in a super high purity gas piping system and a super high vacuum apparatus.
In recent years, the technology for realizing super high vacuum, or the technology for producing a reduced pressure atmosphere of super high purity by making a small flow amount of a predetermined gas flow into a vacuum chamber has become very important. These technologies are widely used in the research of material characteristics, the formation of various kinds of thin films, the production of semiconductor devices, etc., and as a result, although more and more high vacuum degree has been realized, but it is very strongly desired further to realize a reduced pressure atmosphere in which the mixing of impurity elements and impurity molecules has been reduced to the extreme limit.
For example, when it is exemplified with the case of the semiconductor device, the dimension of unit elements has become smaller year by year in order to improve the assembling degree of the integrated circuit, and research and development are actively carried out in order to practically obtain the semiconductor devices having the dimension of 1 .mu.m to submicron, and further, less than 0.5 .mu.m.
The production of such a semiconductor device as described above is carried out by repeating the procedure for forming thin films, the procedure for etching the formed thin films to a predetermined circuit pattern, etc. Then, in the process such as described above, it is usual to carry out the procedure in a super high vacuum state or in a reduced pressure atmosphere in which a predetermined gas has been introduced by putting the silicon wafers in general in a vacuum chamber. If impurities are mixed in these procedures, there are generated problems such as, for example, the film quality of thin films is deteriorated, and the accuracy of the fine finishing can not be obtained. This is the reason why the super high vacuum and the reduced pressure atmosphere of super high purity are requested.
As one of the greatest causes which prevented the realization of the super high vacuum and the reduced pressure atmosphere of super high purity, can be cited the gas released from the surface of the stainless steel and the like which are widely used in the chamber and gas pipings. Especially, it was the greatest pollution source that the water adsorbed on the surface was separated out in vacuum or in the reduced pressure atmosphere.
FIG. 6 is a graph for showing the relationship between the total leak amount of the system added with the gas piping system and reaction chambers in various kinds of apparatus (the sum of the released gas amount from the piping system and the reaction chamber internal surface and the external part leak) and the pollution of the gas. By the way, it is assumed that the original gas does not perfectly contain impurities. Plural number of lines in the figure show the result of the cases in which the flow amount has been changed to various values by making it as a parameter. Although it is a matter of course, that the less the gas flow amount becomes, the more the effect of the released gas from the internal surface is revealed, and the impurity concentration becomes relatively high.
The semiconductor process has such a tendency as to decrease the flow amount of the gas more and more in order to realize the procedures of higher accuracy such as the hole opening, the hole burying, etc. of high aspect ratio, and for example, it is usual, for example, in the process of submicron ULSI to use the flow amount of several ten cc/min or the less. When it is assumed that the flow amount of 10 cc/min is tentatively used, there is the system total leak about 10.sup.-3 to 10.sup.-6 Torr.multidot.l/sec in such a manner as in the apparatus widely used at present, the purity of the gas becomes 1% to 10 ppm, and it becomes far from the one of the high purity process.
The present inventor has invented the supply system of super high purity as, and has succeeded to check the leak amount from the external part of the system to less than 1.times.10.sup.-11 Torr.multidot.l/sec which is the detection limit of the detector at present. However, due to the leak from the inside of the system, that is, due to the constituents of the released gas from the surface of the above-described stainless steel, it was unable to reduce the impurity concentration of the reduced pressure atmosphere. The minimum value of the surface released gas amount obtained by the surface treatment in the super high vacuum technology at present, is in the case of the stainless steel is 1.times.10.sup.-11 Torr.multidot.l/(sec.multidot.cm.sup.2), and even it is assumed that the surface area exposed in the inside of the chamber has been estimated to be smallest such as, for example, 1 m.sup.2, the leak amount becomes in total as 1.times.10.sup.-7 Torr.multidot.l/sec, and the gas of the purity of about 1 ppm can be obtained for the gas flow amount of 10 cc/min. It is needless to say that the purity decreases further, when the gas flow amount has been diminished further.
In order to decrease the degassed constituent from the internal surface of the chamber to about the same degree as 1.times.10.sup.-11 Torr.multidot.l/sec which is equal to that of the external leak amount of the total system, it is necessary to make the degassing from the surface of the stainless steel be less than 1.times.10.sup.-15 Torr.multidot.l/sec cm.sup.2, and for that purpose, the treatment technology of the surface of the stainless steel for decreasing the gas release amount was strongly requested.
Also, in the semiconductor production process, various kinds of gases such as from the comparatively stable general gases (O.sub.2, N.sup.2, Ar, H.sup.2, He) to the special gases having strong reactivity, corrosive properties, and toxicity are used. In general, as the material of the piping and chamber for treating these gas, stainless steel is used in many cases from the reasons such as the reactivity, anti-corrosiveness, high strength, the easiness of the secondary processability, the easiness of the welding, and the easiness of polishing the internal surface.
Stainless steel is excellent in the anti-corrosiveness in a dry gas atmosphere. However, in special gases, there are such ones as boron trichloride (BCl.sub.3), boron trifluoride (BF.sub.3), which show strong corrosiveness by forming hydrogenchloride and fluoric acid when water is present in the atmosphere, and in the case when water is present in the gas atmosphere of the chlorine system and fluorine system such as the above-described BCl.sub.3 and BF.sub.3, stainless steel is easily corroded. Therefore, after the surface polishing of the stainless steel, the anti-corrosiveness treatment becomes indispensable.
As the treatment method for the anti-corrosiveness, there are the Ni-W-P coating and the like method (clean escorting method) which covers stainless steel with a metal having strong anti-corrosiveness, but in these methods, not only cracks and pin holes are liable to be formed, but also there are problems such as that the adsorption amount of water on the internal surface and the residual constituents of the solution become much, since they are the method of using the wet type galvanization. As another method, can be cited the anti-corrosiveness treatment in which a thin oxide film is formed on the metal surface by the inactivation treatment. Since stainless steel is inactivated by only being immersed in a liquid, when there is present a sufficient oxidizer in the liquid, the inactivation treatment is carried out in general at ordinary temperature or in a state where the temperature is somewhat raised in immersing in a nitric acid solution. However, since this method is also a wet method, much water and residual component of the treating solution are present in the piping and on the internal surface of the chamber. In the abovedescribed methods, the presence of adsorbed water on the internal surface gives severe damage to the stainless steel in the case when the chlorine system and fluorine system gases were made flown thereon.
Therefore, it is very important in the super high vacuum technology and the semiconductor process to constitute the chamber and the gas supply system with stainless steel formed with an inactive state film which is not subjected to damage even for a corrosive gas and has little absorption and adsorption of water.
For example, in the inactivation treatment of the stainless steel pipe, when the heat oxidation treatment has been carried out in a high purity atmosphere in which the content of water is less than 10 ppb, an inactive state film excellent in the degassing characteristics can be obtained.
FIG. 7 shows the change of the water amount contained in the purge gas in the case when stainless steel pipes having different internal surface treatment have been purged at ordinary temperature. In the experiment, N.sub.2 gas was flown in a 3/8" stainless steel pipe of the total length of 4 m at the flow amount of 450 ccm, and the water amount contained in the N.sub.2 gas at the outlet was measured by use of the HYCOSMO (low temperature optical dew point measuring instrument).
In FIG. 7, (a) shows the result of the test on the stainless steel pipe having been electrolytically polished on the internal surface.
The test shown in FIG. 7 has been carried out after the sample has been left for about 1 week in a clean room of the relative humidity of 50% and the temperature of 23.degree. C.
As is clearly known in (a) of the FIG. 7, it is known that, in the electrolytically polished pipe, a large amount of water is detected. After passing the gas for about 1 hour, about 100 ppb of water is also detected, and even after 2 hours, the water amount is detected for about 50 ppb, and it is known that the water amount is not quite decreased.
On the contrary, it was elucidated by the present inventor that the process has extremely excellent degassing characteristics in the case when the inactivated state film has been formed in a high purity dry atmosphere.
However, it is necessary to make the water content be less than 10 ppb in order to produce a stainless steel pipe having the extremely excellent degassing characteristics of the adsorbed gas, and in order to realize the super high purity oxidation atmosphere, the condition control of a high degree is necessary, and the process has high cost and its production efficiency is bad, so that the process could not be said as to be suitable to mass production. That is, by use of the conventional generally used metal oxidation treatment apparatus and the metal oxidation treatment method, it was unable to realize the oxidation atmosphere of such a super high purity as described above.
Also, especially in the stainless steel pipes having small internal diameter of such as 1/4", 3/8" and 1/2", the oxidation treatment has been carried out in such a state as it is that the inside of the stainless steel pipe has been exposed to the ambient air atmosphere and has been polluted. Also, since the external side of the stainless steel pipe has in general no relationship to the characteristics, it is very much polluted in comparison to the internal surface. When there is such a case that the gas contacting to this external surface is mixed to the gas for treating the internal surface, it is very difficult to keep the super high purity degree of the gas for treating the internal surface, and an inactivated film of good quality which is excellent in the corrosiveness and having little occlusion and adsorption of water can not be formed. Also, in the external side of the stainless steel pipe, the surface after the oxidation treatment becomes dirty by the roughness and dirtiness of the surface. Due to the fact that the external side of this stainless steel pipe oxidized becomes the cause of problems such that the pipe looks dirty and particles are generated in the case when it was piped in a clean room, together with that an inactivated state film of good quality can not be formed on the internal surface.
Therefore, in the mass production technology of the inactivation treatment of the metals to be oxidized such as the stainless steel pipe and the like, it was desired to establish the technology in which the external surface is not oxidized, together with that an inactive state film having excellent anti-corrosiveness and little occlusion and adsorption of water is formed.
Therefore, the apparatus shown otherwise in FIG. 8 has been proposed as such a technology (Japanese Patent Application No. 195185/1988).
In the apparatus shown in FIG. 8, a groove 134 having the diameter of approximately the same as the external diameter of the stainless steel pipe 101 is formed on the one surface, and the introducing port 135 and exhausting port 136 of the gas are formed on the another surface, and further, a pair of holders 103 and 104 which has communicated the groove 134 to the introducing port 135 and the exhausting port 136 is used, and further, the apparatus has such a structure that an inactive gas is introduced into the oxidation treatment furnace 137 from 119 and can be exhausted from 121.
The stainless steel pipe 101 is inserted into the groove 134 at the end thereof, and are held on the holders 103 and 104. Also, in another surfaces of the holders the gas introducing pipe 107 and the gas exhausting pipe 109 are connected.
That is, as the maximum feature of this technology, in the oxidation treatment furnace 137, while the gas is introduced form one end of the stainless steel pipe 101, ad the gas is always exhausted from another end, and impurities of the water separated from the internal surface of the stainless steel pipe 101 as the metal to be oxidation treated is exhausted out of the oxidation treatment furnace 137, and the stainless steel pipe 101 can be heat oxidized in a dry oxidation treatment atmosphere. Thereby, the water concentration in the oxidation treatment atmosphere can be decreased to less than the value objected (for example, less than 10 ppb in the case of the stainless steel), and the formation of a good inactivated state film on the surface of the metal to be oxidation treated is enabled.
Also, even if it is a stainless steel pipe in which the gas is difficultly flown, such as the stainless steel pipe and the like having small internal diameter, since the inlet and the outlet of the gas are arranged in such a manner that both ends of the stainless steel pipes are contacted, it becomes possible that the oxidation treatment atmosphere gas is flown in the inside of the stainless steel pipe, and the metal processed is heat oxidized in a dry oxidation treatment atmosphere. Thereby, the water concentration in the oxidation treatment atmosphere can be reduced to less than the objected value (for example., less than 10 ppb), and the formation of good inactivated state film on the surface of the metal processed becomes possible.
However, it was understood that the following problems are generated in this technology.
(1) At first, it is difficult to insert the stainless steel pipe 101 in the groove 134 of the holders 103 and 104. That is, when the internal diameter of the groove 134 is made too larger than the external diameter of the stainless steel pipe 101, there is generated a gap between the groove 134 and the stainless steel pipe 101, and the oxidative gas flows into the oxidation treatment furnace 137, and together with that an activated state film of good quality can not be formed on the internal surface of the stainless steel pipe 101, the external surface is also oxidized, and in order to prevent such a phenomenon, it is necessary that the internal diameter of the groove 134 is made approximately the same with the external diameter of the stainless steel pipe 101. However, when the internal diameter of the groove 134 and the external diameter of the stainless steel pipe 101 are made approximately be in the same size, the insertion of the stainless steel pipe in the groove 134 becomes difficult.
Especially, in the case when the stainless steel 101 has long length or has a small diameter, the difficulty is further increased.
Also, it is also difficult to finish the internal diameter of the groove 134 with good accuracy such that it is approximately the same with the internal diameter of the stainless steel pipe 101.
(2) Secondly, even if the finishing of the groove could be finished with good accuracy, in the case when fluctuation is present in the external diameter of the stainless steel pipes, the insertion into the groove 134 becomes impossible, when the external diameter is large, and on the contrary, when the external diameter is small, a gap is generated as has been described above, and together with that an inactivated state film of good quality can not be formed on the stainless steel pipe 101, external surface burning is generated on the external surface. By the way, such an external burning is liable to be generated at the end part of the stainless steel pipe 101.
(3) Since the gap between the holders 103 and 104 of the stainless steel pipe is constant, in the case when fluctuation was present in the length of the stainless steel pipe, then, as shown in FIG. 9, gap is generated between the groove 134 and the stainless steel pipe 101s in the case of a short stainless steel tube 101s, and an oxidative gas flows into the oxidation treatment furnace 137 from the gap thereof, and together with that inactivated state film of good quality can not be formed on the internal surface as described in (1) and (2), and there is generated the external surface burning.
(4) When the elongation by thermal expansion is generated in the stainless steel pipe 101 at the time of heating, deformation is generated in the pipe processed, since its both ends are restricted. When play is made to be present in order to prevent the deformation, the oxidative gas flows into the space of the oxidation treatment furnace from the inlet as described in (3), and together with that an inactivated state film of good quality can not be formed on the internal surface of the stainless steel pipe, but also the external surface is oxidized.
(5) In the case when the stainless steel pipe is a long length pipe, bending due to the weight of itself is generated at the central part.
By the way, the above-described problems were found out by the present inventor, and the present invention has been carried out on the basis of the discovery of such problems.